The life cycle of Plasmodium falciparum (P. falciparum) is characterized by pronounced morphologic changes. These changes are associated with the stage-specific expression of unique proteins on the parasite's surface. Antibodies to the major surface proteins of fertilized zygotes and ookinetes, Pfs25 and Pfs28 respectively, block the morphologic development of the parasite in the mosquito midgut. This indicates that the expression of these proteins is critical to the development of the sexual stages. This information has been utilized in the formulation of transmission blocking vaccines. Definition of the molecular mechanisms responsible for the sexual stage-specific expression of these critical proteins, however, has been hampered by technical difficulties in sequencing the parasite's highly AT- rich intergenic regions and the inability to functionally test putative gene regulatory elements. The success of the malaria genome project and the development of transfection techniques for the malaria parasite now permit the delineation of the molecular mechanisms of sexual stage-specific gene expression. Understanding the basic mechanisms of P. falciparum sexual stage gene expression could suggest novel strategies or methods to block disease transmission. We have utilized a gene transfection system of the sexual stages of the chicken malaria Plasmodium gallinaceum (P. gallinaceum) as a model system for defining the gene regulatory elements of the evolutionarily closely related P. falciparum parasite. Studies of the P. gallinaceum homologue of Pfs28, Pgs28, reveal that a unique T-rich element in conjunction with a eukaryotic polyadenylation consensus signal in the pgs28 3' UTR are required for Pgs28 protein expression. The function of T-rich 3' gene flanking elements in other eukaryotic genes associated with development supports the hypothesis that the coordinated expression of Pfs25 and Pfs28 is mediated by 3' gene flanking regulatory elements. The specific aims of this project are: 1) to determine the molecular mechanism by which the T-rich element regulates Pgs28 expression; 2) to define the molecular mechanisms responsible for the coordinated expression of Pfs25 and Pfs28; 3) to determine the contribution of 3' gene flanking elements in mediating sexual stage-specific gene expression.